The present invention relates to a detent escapement for a timepiece comprising an escape wheel provided with teeth, a balance onto the staff of which rollers are secured including a large roller provided with an impulse pallet stone and surmounted by a first actuating finger-piece, and a small roller in the circular periphery of which a notch is made, and a blocking member in the form of a lever hinged on a pin, said blocking member carrying means for locking the escape wheel, a second actuating finger-piece and a follower ending in a beak arranged for cooperating with a rising edge of the notch of the small roller.
A detent escapement answering the above description has already been proposed in European Patent Application No. 03027929.3, filed on 4 Dec. 2003. However, in that patent application a resilient member acts on one of the actuating finger-pieces—in that case on the first finger-piece forming part of the balance roller—such that the finger-piece operates the release of the blocking member when the roller is rotating in a first direction and keeps said blocking member engaged in the escape wheel when the roller is rotating in a second direction opposite to the first. In this second rotational direction, the finger-piece resiliently attached to the roller retracts when faced with the finger-piece fixed to the blocking member. It should also be mentioned that the escapement of the aforecited patent application, in addition to using the aforementioned resilient member, requires the use of a stop pin for limiting the play of the blocking member and keeping it there in a certain shape while the roller performs its supplementary arc of oscillation.
It will be understood that if the resilient member, on the one hand, and the stop pin fitted to the escapement mentioned in the above paragraph could be omitted, the construction of the assembly would be considerably simplified, and this is the object of the present invention.
A detent escapement free of any resilient members and with no stop pin was proposed in European Patent Application No. 03028877.3 filed on 16 Jan. 2003. In this patent application, the escapement includes a large roller carrying a first finger-piece and a blocking member carrying a second finger-piece and a locking pallet stone. The first and second finger-pieces are shaped such that when the large roller is rotating in a first direction, the first finger-piece drives the second which passes round a first side of said first finger-piece to release the locking pallet stone from the escape wheel. Re-engagement occurs when the second finger-piece scales a rising edge of a notch made in a small roller. When the large roller is rotating in a second direction opposite to the first, the first finger-piece drives the second finger-piece which passes round a second side, opposite the first side, of said first finger-piece to hold the locking pallet stone in the escape wheel.
It will be understood here that the shocks created when the first and second finger-pieces meet exert a perpendicular force to the rotational axis of the blocking member. Moreover, the finger-pieces in question are made of edges that could enter into collision and break in the mechanism if the latter is incorrectly adjusted. Improved reliability is another object of the present invention.